1. Technical Field
The present invention relates to an optical system of an atomic oscillator, in particular, relates to a structural technique by which an S/N ratio of transmitted light emitted from a gas cell included in the atomic oscillator is improved.
2. Related Art
Atomic oscillators using alkali metals such as rubidium and cesium operate while maintaining a gas cell, in which atoms are air-tightly sealed, at a high temperature because the atoms need to be kept in a gas state when the oscillators use energy transition of the atoms. An operating principle of the atomic oscillators is broadly classified into a double resonance method utilizing light and micro waves and a method utilizing quantum interference effect produced by two kinds of laser light (hereinafter, referred to as coherent population trapping: CPT). In both of the methods, atomic resonance is detected by detecting how much light made incident on the gas cell is absorbed by atom gas with a detector, so as to allow a reference signal of a quartz crystal oscillator and the like to synchronize with the atomic resonance by a control system, obtaining an output. The detector is disposed at an opposite side to a side, to which the light is made incident, of the gas cell. However, in the CPT method, when resonance light has a wavelength at which the light is absorbed by the gas cell, metal atoms in the gas cell emit fluorescence due to the light absorbed by the gas cell. The fluorescence is incident on the light detector as leaked light, so that a level of primary transmitted light is increased, resulting in degradation of an S/N ratio of a detecting signal.
U.S. Pat. No. 6,265,945 B1, for example, discloses an atomic oscillator employing the CPT method by which part of light of a light source split by a beam splitter is detected so as to stabilize power of the light source.
Referring to FIG. 6, in a structure of a related art optical system disclosed in the above example, laser light emitted from a vcsel 50 is split by a beam splitter 53. One beam of the laser light is detected at an optical pwr. detector 52, and the other beam of the laser light is transmitted through a cell 55 and is detected at a transmission detector 56. Fluorescence is emitted from the cell 55 and a signal complementary with a signal detected by the transmission detector 56 is detected by a fluorescence detector 54. However, the related art has no blocking unit blocking emitted fluorescence. Thus the related art has not solved the problem of the degradation of an S/N ratio of the transmitted light.